In thread cutters of the aforedescribed type, an internal thread is provided on workpieces, for example prebored nuts, while they are held against rotation but are allowed to move over the thread-cutting region onto the smooth-surfaced shaft. Thus after the generation of the thread in the workpiece, the now-threaded workpiece passes from the end of the thread cutter or tap onto the shaft from which the individual workpieces or nuts can be thrown.
Conventional thread cutting machines of this type can operate at a cutting speed of 75 m/minute and at rotary speeds, for example, for M8 nuts (utilizing the European designation system) of 3000 rpm. While higher cutting speeds are possible, rotary speeds in excess of 3000 rpm could not be exceeded prior to the present invention without creating significant difficulties in the manner in which the nuts are cast off the shaft centrifugally.
Various configurations of the shafts used in prior art machines are known. In most cases, a shaft extending at a right angle to the thread cutter portion has been used. With such systems, there is the disadvantage that the thread cutting member cannot be rotated precisely coaxially with the bore of the article to be threaded and centrally thereof, because the force application to the rotary parts can occur only at the outer ends of the curved shaft so that the latter, in its rotary movement, will describe the surface of cone whose apex angle and dimensions will be dependent upon the difference between the shaft diameter of the thread cutter and the basic bore in the part to be threaded as well as the difference between the outer diameter of the part to be internally threaded and the inner diameter of its guide, for example, its guide sleeve, for that portion of the latter already filled with threaded workpieces.
In practice, therefore, the thread which is fabricated is always larger than the diameter of the thread cutter or tap. Maintenance of narrow thread tolerances is practically impossible.
A further drawback of such pass-over tapping machines is that the workpiece which can pass along the curved shaft can have only a limited height or axial dimension, i.e. a height of at most 1.5 times the thread diameter to prevent binding of the workpiece in its travel along the shaft. Greater axial lengths or heights may result in catching of the workpiece on the shaft and a failure of the shaft to adequately cast off the individual workpieces.
During the centrifugal cast-off action, an axial counterforce is applied to the workpieces which remain on the thread cutter so that during the cutting operation there is a higher flank pressure which can result in excessive tool wear.
The radial discharge of the finished workpieces from the angled shaft is effected with such high outward velocity that the workpieces which have previously collected in the basket and the workpieces flung off from the shaft may be damaged by impact with one another.
Finally it may be mentioned that a relatively large back and forth movement of the tap is required in these conventional machines to ensure reliable operation and that is disadvantageous with respect to the production rate.
Pass-over types of tapping machines are also known in which the shaft at the end is initially radially outwardly curved and then provided with a radially inward curvature. These devices have not been found to be satisfactory in practice because additional forces are applied to the shaft and the thread cutter, at least in part because it is then necessary to displace the threaded workpieces counter to the centrifugal force. Here again the tool wear is inordinately increased and the dimensional tolerances within which the threads are fabricated can become excessive. As a result, tapping machines of this type are practically no longer in use.